Many types of photosensitive detectors, such as photodiodes and phototransistors, exist in the prior art. One class of photosensitive detectors includes transmissive and transparent detectors. Transmissive detectors are detectors that are capable of transmitting at least a portion of incident electromagnetic (“EM”) energy therethrough. Transparent detectors are a subset of transmissive detectors, further restricted to photosensitive detectors that are capable of transmitting EM energy therethrough with little absorption and no appreciable scattering or diffusion.
U.S. Pat. No. 6,037,644, entitled SEMI-TRANSPARENT MONITOR DETECTOR FOR SURFACE EMITTING LIGHT EMITTING DEVICES, U.S. Pat. No. 6,879,014, entitled SEMITRANSPARENT OPTICAL DETECTOR INCLUDING A POLYCRYSTALLINE LAYER AND METHOD OF MAKING (hereinafter, the “'014 patent”) and U.S. Pat. No. 6,670,599, entitled SEMITRANSPARENT OPTICAL DETECTOR ON A FLEXIBLE SUBSTRATE AND METHOD OF MAKING, are incorporated herein by reference in their entireties. Each of these patents describes a semitransparent detector for monitoring the performance of monochromatic optical devices such as a vertical cavity surface-emitting laser (“VCSEL”). The semitransparent detectors described in the aforementioned publications are non-imaging detectors designed to respond to a single wavelength of EM energy and be integrated into the optical device. The '014 patent does note that it is difficult to design and fabricate a transparent detector that simultaneously exhibits good responsivity at a desired wavelength, has low dark current and is highly transparent; however, this document does not discuss possible solutions to these problems outside of the art of VCSEL monitoring.
FIG. 1 is an illustration of a prior art thin film detector 100 that is representative of FIG. 1 of the '014 patent. In accordance with the descriptions of the '014 patent, detector 100 may be utilized for receiving electromagnetic energy that propagates along a light path 10 that is incident, for example, on the upper (exposed) surface of a partially conductive layer 120. Detector 100 may be configured as a semitransparent detector, and may be arranged transversely with respect to light path 10 such that the detector receives at least some of the electromagnetic energy. In other words, the detector can be at least partially transparent with respect to the incident electromagnetic energy, such that a first portion of the incident electromagnetic energy may be absorbed and detected, and a second portion that is not absorbed continues along light path 10 for subsequent use. In thin film detector 100, a photosensitive layer (i.e., a detecting medium) 110 is formed, for instance, from silicon, germanium or other appropriate semiconductor material. At least partially transparent conductive layers 120 and 130 are formed upon two sides of photosensitive layer 110 and may be patterned so that only portions of photosensitive layer 110 are contacted by conductive layers 120 and 130. Conductive layers 120 and 130 are formed from a material such as indium tin oxide (“ITO”), zinc oxide, tin oxide or zinc/tin/indium mixed oxide and are configured to provide electrical contact with photosensitive layer 110. Photosensitive layer 110 and conductive layers 120 and 130 are supported on a substrate 140 that serves as a support arrangement having a support surface over which the aforementioned layers, including photosensitive layers 110, may be formed. Using well-known techniques substrate 140 may be, for example, formed from a transmissive dielectric material such as glass or silicon dioxide. Photosensitive layer 110 may include a number of sublayers (not shown) such as a transmissive p-type layer (P), an intrinsic (e.g., undoped) semiconductor (I), and an n-type semiconductor (N), and these layers may be configured according to well known techniques as a “PIN” diode for absorbing light and producing an electrical signal based on the absorbed light. As discussed in the '014 patent, the sublayers may be formed by sequential deposition of each layer, and/or selective masking and doping of one or more deposited semiconductor materials. While thin film detectors can be relatively inexpensive to fabricate, they often suffer from excessive noise, large dark currents and low sensitivity particularly when amorphous semiconductor materials are used to form the PIN diode.
As described in the '014 patent, the EM energy may propagate along light path 10 that is oriented in an upward direction, and may therefore be incident on the lower surface (not shown) such that it may be received by the detector.
FIG. 2 is an illustration of a prior art thinned detector 200. Thinned detector 200 may include sub-layers of thinned crystalline material and, like thin film detector 100 (FIG. 1), can be configured to form a PIN diode. In particular, thinned detector 200 may include a p-type semiconductor substrate 210 that can be thinned by mechanical and/or chemical means, supporting an intrinsic semiconductor layer 220 and an n-type semiconductor layer 230 formed thereon. P-type semiconductor substrate 210 can be formed from a thick (e.g., hundreds of microns) crystalline semiconductor substrate that is chemically and/or mechanically thinned (e.g., to a final thickness of about 10 microns or less) to provide appropriate optical characteristics for a given application. Thinned detector 200 may be lithographically patterned and etched, using well-known techniques, to remove portions of layers 220 and 230 to accommodate a p-contact 240, which provides an electrical contact with p-type semiconductor substrate 210. An n-contact 250 can be deposited onto n-type semiconductor layer 230. P-contact 240 and n-contact 250 can be formed from aluminum or other suitable conductive materials known in the art. The thinning operation itself, as well as yield losses due to damage of thinned substrates, may contribute to a higher final cost of a thinned detector as compared to the final cost of a thin film detector such as that shown in FIG. 1.
FIG. 3 shows an exemplary plot 300 of simulated spectral performance associated with one example of prior art thin film detector 100 fabricated according to FIG. 1 and the description thereof. Plot 300 may, for example, be generated based on the device specifications disclosed in the '014 patent, and has wavelength in nanometers as the abscissa and percentage as the ordinate. Overall reflectance 310 of thin film detector 100 is represented by a dashed line, and absorptance 320 of photosensitive layer 100 alone is represented by a dash-dot line. Transmittance 330 through thin film detector 100 is represented by a dotted line, and absorptance 340 of all other layers of thin film detector 100 other than photosensitive layer 110 is represented by a solid line. Based on plot 300, thin film detector 100 may be considered inefficient since an average value of reflectance 310 is greater than 50%. Furthermore, an average value of absorptance 320 is less than 30% and exhibits pronounced spectral dependence due to material properties and solid etalon effects in the detector structure.
Although thin film detector 100 and thinned detector 200 may provide adequate detection for EM energy of a single wavelength for a given application, such as monochromatic optical device monitoring, their spectral dependence (as demonstrated, for example, in FIG. 3) makes them unsuitable for uniform broadband detection. Additionally, loss of incident EM energy due to reflection contributes to reduced signal-to-noise ratio (“SNR”) of the thin film and thinned detectors.
Outside the art of monochromatic optical device monitoring, an application for a thin film silicon detector includes a highly transparent phase detector that detects the EM energy intensity along standing waves for high precision measurement of position or wavelength. Examples of such applications are described in Jun et al., “Optimization of Phase-Sensitive Transparent Detector for Length Measurements,” IEEE Trans. Electr. Dev., 52, No. 7 (2005), pp. 1656-1660, Li et al., “Precision optical displacement sensor based on ultra-thin film photodiode type optical interferometer,” Meas. Sci. & Tech., 14 (2003), pp. 479-483 and Knipp et al., “Silicon-Based Micro-Fourier Spectrometer,” IEEE Trans. Electr. Dev., 52, No. 3 (2005), pp. 419-426, which are incorporated by reference herein in their entireties. Transparent phase detectors of these types are designed to minimally impact the incident EM energy and therefore detect only a very small fraction of the incident energy; that is, such transparent phase detectors are intended to absorb only as little incident EM energy as needed to perform the phase detection task. While the detectors described in the foregoing documents (including the '014 patent) may be configured to absorb and detect a first portion of incident electromagnetic energy, and to transmit a second portion for subsequent use, those detectors are configured to absorb a fraction of overall power of the incident radiation. Therefore, such detectors absorb the first portion of the incident electromagnetic radiation in a manner that is not selective with respect to specific characteristics of the electromagnetic energy. While these prior art detectors may exhibit well known spectral characteristics based on material properties and other conventional features of the detector, the absorption characteristics of the foregoing detectors are not selective with respect to specific characteristics of the electromagnetic energy such as wavelength and/or polarization.